Can't select Edge loop (Alt + RMB is failing )
While in Edit Mode, you have the ability to limit the kinds of selections that your cursor can make.



When I took this screenshot it was set to Vertex selection mode here, the two others are Edge selection and Face selection.


When in Vertex selection mode you can't pick edges or faces directly.
When in Edge selection mode you can't pick vertices or faces directly.
When in Face selection mode you can't pick edges or vertices directly, as a consequence this prevents edge loops from being selected


You can have multiple modes active at the same time, which can be handy, by holding down shift and click on the icon.